This invention relates generally to medical imaging systems and more particularly to systems and methods for compensating for table sag.
In at least some computed tomography (CT) imaging system configurations, an x-ray source projects a fan-shaped x-ray beam which is collimated to lie within an X–Y plane of a Cartesian coordinate system and generally referred to as an “imaging plane”. The x-ray source is coupled to a gantry 20 shown in FIG. 1. The x-ray beam passes through an object, such as a patient, being imaged. The object is located on a table 22 that is slidably located on a base 24. The x-ray beam, after being attenuated by the object, becomes an attenuated beam that impinges upon a detector array. Intensity of the attenuated beam received at the detector array is dependent upon the attenuation of the x-ray beam received by the object. Each detector element of the detector array produces a separate electrical signal that is a measurement of the attenuation at a location of the detector array. The attenuation measurements from all the elements of the detector array are acquired separately to produce a transmission profile.
The x-ray source and the detector array are rotated with gantry 20 within the imaging plane and around the object to be imaged, so an angle at which the x-ray beam intersects the object constantly changes. The x-ray source typically includes an x-ray tube, which emits the x-ray beam at a focal spot. A detector element of the detector array typically includes a collimator for collimating attenuated beams received at the detector array, a scintillator adjacent the collimator, and a photodetector adjacent to the scintillator. A group of x-ray attenuation measurements or projection data from the detector array at one gantry angle is referred to as a “view”. A “scan” of the object includes a set of views made at different gantry angles, or view angles, during one revolution of the x-ray source and the detector array.
Although a design of table 22 is capable of providing required accuracy in a z direction parallel to a z axis under heavy loading conditions, table 22 does not maintain a rigidity to provide a location accuracy in a y direction parallel to a y axis. Lack of the rigidity generates a table sag 26. Table sag 26 occurs when table 22 is not fully retracted away from gantry 20 in the z direction. Table 22 is fully retracted when table 22 cannot be retracted further away from gantry 20 in the z-direction. Table sag 26 occurs when table 22 is extended towards gantry 20 in the z direction. Table 22 is fully extended when table 22 cannot be extended further toward gantry 20 in the z direction.
An effect of table sag 26 is that anatomies of the object scanned with table 22 extended is shifted downward as compared to the anatomies that are scanned with table 22 retracted. In radiotherapy (RT) or alternatively positron emission tomography (PET) applications, a registration of an anatomy of the object is important. For example, in RT applications, a radiation treatment planning is performed by identifying a tumor location of a tumor in images generated by using one of the CT imaging system configurations and adjusting a radiation beam appropriately so that an area of the object on which the tumor is located is exposed to the beam and no other areas of the object are exposed. When table sag 26 occurs, however, the tumor location in the images can be several millimeters away from the area of the object in which the tumor is located. Table sag 26, therefore, leads to suboptimal treatment of the object.
FIG. 2 shows an embodiment of a plurality of images 50 and 52 generated by scanning a set of phantoms at two table locations. A first of the two table locations corresponds to a table 54 fully retracted. A second of the two table locations corresponds to table 54 extended out by an amount, such as 1090 millimeters. Sizes of the phantoms placed on table 54 correspond roughly to a small size body. Both images 50 and 52 are reconstructed with the same field of view. Ideally, the two table locations in both images 50 and 52 should be identical. However, because of table sag, table 54 in image 52 is lower than table 54 in image 50, as shown by a discontinuity 56 of table 54 at a boundary 58 between images 50 and 52. Numerical measurement indicates that table 54 in image 52 is shifted by a perpendicular distance of 4.5 millimeters compared to table 54 in image 50. Under heavier loading, table sag 26 can be expected as much as 6 millimeters.